With the development of display technology, display devices with large dimensions, high quality, and low cost become a tendency. Color filter is one of the key components of a display device (such as a TFT-LCD display device), and its quality directly determines the display effect of the display device.
The most critical step in the process of manufacturing TFT-LCD is to expose an array substrate and a color filter plate. Patterns are generated in the product within predetermined regions according to the actual size of a photomask used by a platform of the exposure machine.
Under ideal conditions, patterns with the same design on each part of a glass substrate should have the same dimension after exposure. The dimensions are equal to a design value of the photomask. In order to keep the characteristics of the components identical, the designs for all patterns on the display area of a TFT-LCD panel are usually the same. However, in an actual exposure process, considering the mounting manner of the photomask on the platform of the exposure machine, certain degree of bending deformation of the photomask due to its own gravity might occur. The larger the dimension of the photomask is, the higher the deformation degree. Compared with the perfect condition of no bending deformation in the photomask, the photomask deformation will result in differences between the optical length of exposure from the center of the photomask to the glass substrate and the optical length of exposure from the periphery of the photomask to the glass substrate. This will further lead to different exposure intensities among different parts of the glass substrates corresponding to the different parts of the photomask, thus finally causing the dimensions of the patterns on the glass substrate to be different from each other.
For example, with a horizontal direction as a reference direction, when the center of the photomask concaves downwardly (i.e., the photomask is deformed as a concave mirror), the exposure area of the part corresponding to the center of the photomask will become small, and the exposure area of the parts corresponding to the periphery of the photomask will become large. In contrast, when the center of the photomask protrudes upwardly (i.e., the photomask is deformed as a convex mirror), the exposure area of the part corresponding to the center of the photomask will become large, and the exposure area of the parts corresponding to the periphery of the photomask will become small. The differences in exposure areas indicates different light transmitting areas between the center and the periphery of the liquid crystal display panel, thus resulting in uneven brightness and poor display quality of the liquid crystal display panel. Similarly, for an array substrate, large difference in pattern dimensions is directly manifested in the non-uniform driving performance of TFTs (thin film transistors), thus causing poor display quality of the liquid crystal display panel.